NXP Semiconductors

17

34845

6.2.2

Input capacitor

The input capacitor should handle at least the following RMS current.

6.2.3

Output capacitor

For the output capacitor selection the transconductance should be taken in consideration.

The output voltage ripple (∆VOUT) depends on the ESR of the Output capacitor. For a low output voltage ripple, it is recommended to use

ceramic capacitors which have a very low ESR. Since ceramic capacitor are costly, electrolytic or tantalum capacitors can be mixed with

ceramic capacitors for a less expensive solution.

The output capacitor should at least handle the following RMS current.

6.2.4

Network compensation

Since this Boost converter is current controlled, a Type II compensation is needed. Note that before calculating the network compensation,

all boost converter components need to be known. For this type of compensation it is recommended to push out the Right Half Plane Zero

to higher frequencies where it does not significantly affect the overall loop.

The crossover frequency must be set much lower than the location of the Right half plane zero:

Since the system has a fixed slope compensation, RCOMP should be fixed for all configurations, i.e. RCOMP = 2.0 kΩ

CCOMP1 and CCOMP2 should be calculated as follows:

The recommended values of these capacitors for an acceptable performance of the system in different operating conditions are

CCOMP1 = 33 nF and CCOMP2 = 220 pF.

A resistor network can be implemented from the PWM pin to ground with a connection to the compensation network, to improve the

transient response of the boost. This configuration should inject a 1.0 V signal to the COMP pin and the equivalent Thevenin resistance

of the divider should be close to RCOMP, (i.e. for 2.0 kΩ COMP resistor, RCOMP = 3.3 kΩ and RSHUNT = 10 kΩ. See Figure 10 and

Figure 11 for implementation guidelines.

If a faster transient response is needed, a higher voltage (e.g. 1.3V) should be injected to the COMP pin; so the resistor divider should be

modified accordingly, but keeping the equivalent Thevenin resistance of the divider close to RCOMP.

IRMS C

IN

–

VIN VOUT VIN

–

()

×

2L

×

FSW

×

VOUT

×

---------------------------------------------------------







0.3

×

=

COUT

RCOMP 5

×

GM

×

IOUT

×

L

×

1D

–

() VOUT

×

0.35

×

-------------------------------------------------------------------------------

=

ESRC

OUT

VOUT ΔVOUT

×

FSW

×

L

×

VOUT 1D

–

()

×

---------------------------------------------------------------------------

=

IRMS C

OUT

–

IOUT

D

1D

–

-------------

×

=

fRHPZ

VOUT 1D

–

()

2

×

IOUT 2π

×

L

×

---------------------------------------------

=

fCROSS

fRHPZ

5

---------------

=

CCOMP1

2

π fCROSS

×

COMP

R

×

-------------------------------------------------------------

=

CCOMP2

2GM

6.28 FSW

×

-----------------------------

=